The use of plaster molds in casting of nonferrous metals has long been well-known in the art. Plaster may easily be conformed to a desired pattern to be reproduced as a metal product, the plaster being hardened into a mold, usually in several sections. Molten metal is poured into the mold, allowed to cool and solidify, and the mold is removed from the casting.
The recognized advantages of plaster casting of metals include the low cost of both the plaster molding composition and the moldmaking process, and the ease with which high definition and accurate reproduction of pattern surfaces may be achieved using a plaster composition of suitable consistency. Despite reasons favoring plaster casting of metals, there are several recognized disadvantages to such processes as conventionally practiced. Plaster mold quality is dependent upon a number of variables which must be controlled, including consistency of the plaster molding composition, mold pouring procedures, and plaster curing techniques. Improper procedures may result in blow outs of molten metal from the casting mold, distorted cast metal product shapes due to shrinkage of the plaster mold during curing, rough surfaces of castings due to excessive plaster mold porosity, and complete failure of the metal casting process due to cracking of the plaster mold during the metal pouring step.
The need continues in the art for improvement in the quality of plaster compositions for producing metal casting molds, so that this convenient method of metal casting may be more easily employed. The plaster compositions of the present invention, which comprise insoluble calcium sulfate anhydrite whisker fibers resulting in metal casting molds of improved quality, are a significant contribution to this need.